import java.util.*;

public class Test {



    //火星词典
    Map<Character,Set<Character>> edges = new HashMap<>();
    Map<Character,Integer> in = new HashMap<>();
    boolean check;

    public String alienOrder(String[] words){
        // 1. 初始化⼊度哈希表 + 建图
        for(String s : words)
        {
            for(int i = 0; i < s.length(); i++)
            {
                char ch = s.charAt(i);
                in.put(ch, 0);
            }
        }
        int n = words.length;
        for(int i = 0; i < n; i++)
        {
            for(int j = i + 1; j < n; j++)
            {
                add(words[i], words[j]);
                if(check == true) return "";
            }
        }
        // 2. 拓扑排序
        Queue<Character> q = new LinkedList<>();
        for(char ch : in.keySet())
        {
            if(in.get(ch) == 0) q.add(ch);
        }
        StringBuffer ret = new StringBuffer();
        while(!q.isEmpty())
        {
            char t = q.poll();
            ret.append(t);
            if(!edges.containsKey(t)) continue;
            for(char ch : edges.get(t))
            {
                in.put(ch, in.get(ch) - 1);
                if(in.get(ch) == 0) q.add(ch);
            }
        }
        // 3. 判断
        for(char ch : in.keySet())
        {
            if(in.get(ch) != 0) return "";
        }
        return ret.toString();
    }

    public void add(String s1, String s2)
    {
        int n = Math.min(s1.length(), s2.length());
        int i = 0;
        for( ; i < n; i++)
        {
            char c1 = s1.charAt(i), c2 = s2.charAt(i);
            if(c1 != c2)
            {
                // c1 -> c2
                if(!edges.containsKey(c1))
                {
                    edges.put(c1, new HashSet<>());
                }
                if(!edges.get(c1).contains(c2))
                {
                    edges.get(c1).add(c2);
                    in.put(c2, in.get(c2) + 1);
                }
                break;
            }
        }
        if(i == s2.length() && i < s1.length()) check = true;
    }









    //课程表II
    public int[] findOrder(int n,int[][] prepequisites){
        int[] in = new int[n];
        List<List<Integer>> edges = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            edges.add(new ArrayList<>());
        }

        for (int i = 0; i < prepequisites.length; i++) {
            int a = prepequisites[i][0] , b = prepequisites[i][1];//b->a
            edges.get(b).add(a);
            in[a]++;
        }

        Queue<Integer> q = new LinkedList<>();
        int[] ret = new int[n];
        int index= 0;
        for (int i = 0; i < n; i++) {
            if (in[i]==0) q.add(i);
        }

        while(!q.isEmpty()){
            int t = q.poll();
            ret[index++] = t;
            for (int a :edges.get(t)) {
                in[a]--;
                if (in[a] == 0) q.add(a);
            }
        }
        if (index == n) return ret;
        return new int[0];
    }





    //课程表
    public boolean canFanish(int n,int[][] p) {
        int[] in = new int[n];
        Map<Integer, List<Integer>> edges = new HashMap<>();

        for (int i = 0; i < p.length; i++) {
            int a = p[i][0], b = p[i][1];//b->a
            if (!edges.containsKey(b)) {
                edges.put(b, new ArrayList<>());
            }
            edges.get(b).add(a);
            in[a]++;
        }

        Queue<Integer> q = new LinkedList<>();
        for (int i = 0; i < n; i++) {
            if (in[i] == 0) q.add(i);
        }

        while (!q.isEmpty()) {
            int t = q.poll();
            for (int a : edges.getOrDefault(t, new ArrayList<>())) {
                in[a]--;
                if (in[a] == 0) q.add(a);
            }
        }

        for (int x : in) {
            if (x != 0) return false;
        }
        return true;
    }
}
